Strength, stiffness, resonance and the design of offshore wind turbine monopiles

• Delineation of whether strength or stiffness controls the design of monopiles.
• Six cases considered, three sites on U.S. Atlantic coast and two mudline conditions.
• Stiffness found to control monopile design for two of the six cases.
• 6–8% reduction in monopile mass if stiffness did not control these two cases.
• Strength under operational conditions found to control design for four of the six cases.

A monopile supporting an offshore wind turbine (OWT) is currently designed for both strength and stiffness. Regarding strength, the monopile is designed to have sufficient capacity to withstand demands under both 50-year operational conditions, when the rotor is spinning and blades are oriented to optimize power generation, and 50-year extreme conditions, when the rotor is parked and the blades feathered to minimize aerodynamic loads. Regarding stiffness, the monopile is designed to have sufficient stiffness such that the first structural frequency of the OWT falls between the 1P and 3P frequencies (rotation frequency and blade passing frequency for a three-bladed turbine). For six case studies, including three sites along the U.S. Atlantic coast and two mudline conditions (fixed and compliant), this paper delineates the conditions under which stiffness and strength govern the design of the monopile. This distinction has important implications for the overall risk profile of an OWT, as monopiles controlled by stiffness will have more reserve capacity than monopiles controlled by strength. The six case studies are intended to consider a range of water depths, metocean environments and mudline conditions that is representative of conditions suitable for installing OWTs supported by monopiles along the U.S. Atlantic coast. The monopile designs are controlled by stiffness for two of the six cases studies and, for these two cases, a modest (6–8%) reduction in monopile area (and mass) could be achieved if dynamic design requirements were achieved through means other than increasing monopile stiffness. Monopile designs for the remaining four cases are controlled by operational moment demands.